Broad-band surface optical coupler based on a SiO₂-capped vertically curved silicon waveguide

A chip-surface optical coupler based on a vertically curved Si waveguide was demonstrated for coupling with high-numerical-aperture single-mode optical fibers with a mode-field diameter of 5 µm. This device features a dome-like SiO₂ coupler cap, which acts as collimation lens. We succeeded in fabricating this structure using an isotropic SiO₂ deposition technique employing plasma-enhanced chemical vapor deposition and obtained a light output that approximates that of a 5-µm-waist Gaussian beam. The fabricated coupler showed a coupling loss of less than 4.2 dB and a 0.5-dB-loss bandwidth above 150 nm for TE-polarized light.